In the construction of a typical heat exchanger, a large number of tubes must pass through a tube sheet, and leak-proof joints must be formed between the tubes and the sheet. When, for example, the heat exchanger forms part of a nuclear power plant, unusually high standards of reliability are required. In such applications, the tube sheet, which is made of steel as much as two-feet thick, separates heated radio-active nuclear material from non-radio-active cooling fluid, such as water. It is apparent that even very small leaks between these heat exchanger zones are intolerable. A large number of such joints are included in a single heat exchanger, and each joint must meet the same high standards of reliability.
Although roller swaging has been used to form tube and tube sheet joints, hydraulic swaging has proven to be superior. Hydraulic swaging pressures as high as 50,000 p.s.i. can be uniformly applied throughout a selected axial portion of the tube. In pending U.S. patent application Ser. No. 271,373, entitled: APPARATUS FOR SUPPLYING AND CONTROLLING HYDRAULIC SWAGING PRESSURE, the present inventor has disclosed an apparatus for supplying and controlling hydraulic swaging pressure, and that application is hereby incorporated by reference herein. The invention disclosed in that application permits the automation, to a considerable extent, of the formation of tube/tube sheets joints, reducing the possibility of human error.
In most previously known hydraulic swaging apparatus, a mandrel is inserted in the portion of the tube within the tube sheet, and axially separated seals carried by the mandrel define a pressure zone in which the swaging pressure is to be applied. Pressurized fluid is then introduced through the mandrel into a small annular space between the mandrel and the tube to expand the tube radially. Typically the pressure is first generated by a pump and then multiplied by an intensifier before it is supplied to the mandrel.
A skilled worker must insert the mandrel in each tube individually and cause pressure to be applied by the operation of a control valve. Once the valve has been opened, sufficient time must be allowed for the pressure to reach the desired level. For best results, the pressure should be held at that level for a finite time period on the order of magnitude of two seconds. The optimum swaging pressure and dwell time varies, depending on the specific characteristics of the tube and the tube sheet.
The intensifier is capable of expelling into the mandrel only a relatively small quantity of high pressure fluid before the pistons reach the ends of their strokes. Thus, the mandrel and the interior portion of the tube with which it communicates should be prefilled at low pressure to consistently obtain optimum joints. Otherwise, the pressure generated by the intensifier may be lost in filling the mandrel and the joint will not be properly formed.
Ideally, the swaging apparatus should be automated to reduce the possibility of human error. A primary objective of the present invention is to provide a swaging apparatus for use in forming tube and tube sheet joints which is automated to ensure, with a high degree of reliability, proper prefilling before the increased pressure from the intensifier is applied to the joint. A still further objective is the provision of such an apparatus that is highly efficient and permits each of many joints to be formed consistently within a minimum time period.